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FHWA Highway Safety Programs

Continuous Pavement Friction Measurement (CPFM)

The friction provided by a roadway surface affects how vehicles interact with the roadway. Measuring, monitoring and maintaining pavement friction can prevent many roadway departure and intersection related crashes, resulting in fewer serious injuries and fatalities.

More than 50 years ago, National Cooperative Highway Research Program (NCHRP) Report 37 stated that "the lowest friction levels are found on high-speed roads, curves and approaches to intersections; in short, in locations at which high friction values are needed most." ssentially, this research recognized that friction varies as you travel down the road and that a clear friction "supply and demand" relationship exists, and is a factor in determining the safety performance of a road. The current state of practice for high speed friction measurement by State DOTs is the locked wheel skid trailer which is a discrete, sample-based approach, and thus does not effectively differentiate the changes in friction along the route corridor.

CPFM is an established and proven approach that has been used for several decades in other countries, and could revolutionize the role of pavement friction in framing our understanding and management of the safety performance of our Nation’s roads. This approach is commonly used by road authorities in many European countries, Australia and New Zealand, and even by airport authorities in the U.S. to measure friction on runways. CPFM equipment is able to measure pavement friction continuously, through tangents, curves and intersections.

A recent study by FHWA confirm that CPFM data, combined with crash data and road characteristics, provide significant insight regarding whether friction improvements may reduce crashes. FHWA encourages the use of CPFM to provide comprehensive pavement friction data, combined with existing safety data and analysis, to create an overall pavement friction management program anchored in safety.

Pavements must have sufficient available friction to allow a driver to properly execute a maneuver, especially those that involve steering, braking or acceleration. In 2021, FHWA added Pavement Friction Management, which involves measuring, monitoring, and maintaining pavement friction, to the list of Proven Safety Countermeasures because it can help prevent roadway departure, intersection, and pedestrian-related crashes. Additionally, the integration of pavement friction management and safety management practices in order to achieve safety performance goals is consistent with the Safe System approach. 

A newly published report titled: Characterizing Road Safety Performance using Pavement Friction FHWA-SA-23-006 documents the development of safety performance functions (SPFs) that include friction and macrotexture on a variety of roadway facility types and categories (i.e., segments, intersections, curves, and ramps). The main objectives of the report were: (1) the development of Crash Modification Factors (CMFs), or Crash Modification Functions (CMFx) that make it possible to evaluate the effect of pavement friction changes on safety performance, which can then inform the cost effectiveness of pavement friction improvements; and (2) the establishment of performance or investigatory thresholds for friction based on roadway type and category. The analysis confirmed a strong statistical association between pavement surface frictional properties (friction and macrotexture) and crash rates; lower crash rates were observed with higher friction and macrotexture. 

The findings from this report support road agency efforts toward the institutionalization of Pavement Friction Management, one of the FHWA Proven Safety Countermeasures. The results may be used by road agencies to inform safety analyses at both the system/network and site/project levels to evaluate the impact and cost-effectiveness of pavement friction enhancement strategies and treatments.

In addition, a newly published Primer titled: Pavement Friction for Road Safety: Primer on Friction Measurement and Management Methods FHWA-SA-23-007 was developed to serve as a resource for road safety, maintenance, materials, and pavement engineers and practitioners to provide a basic understanding of pavement surface friction in order to integrate pavement friction into both pavement asset management and road safety management practices. The primer covers pavement friction characteristics such as microtexture, macrotexture, and pavement friction demand. It also discusses different methods of measuring friction and macrotexture, including using continuous pavement friction measurement for collecting friction data at the network-level for safety performance analysis.